doc: add webfonts and tweak the styles accordingly

- Use Fira Sans for headlines, Heuristica for the body (Adobe Utopia derivative). Both are licensed under the SIL OFL license. (I didn't include BoldItalic because it is sparingly used.)
- Split TOC into 2 columns for the docs except manual (too tall, too wide to be readable).
- A few tweaks

Two examples: [modified tutorial](http://adrientetar.legtux.org/cached/rust-docs/tutorial.htm) and [modified manual](http://adrientetar.legtux.org/cached/rust-docs/manual.htm).

**Blocked on graydon/rust-www#25 (for hosting of the fonts), that's showcased [here](http://adrientetar.github.io/rust-www/).**

cc @brson